I prefer to have two lights, one on either side of the mirror - this provides a more asthetically pleasing light with fewer shadows. The size of the mirror can make a big difference, especially if it is part of a cabinet that needs to open. You don't want to limit the style of lamp used too much, but having it too high or low can be a long-term frustration or compromise. During a remodel, it's safer to choose both the mirror and lamps prior to placing the rough-in wiring but you may not have the chance - then, it is a compromise.